by
Bittinger, Marvin L.; Ellenbogen, David J.; Johnson, Barbara L.

Answer

approximately 4 million volunteers

Work Step by Step

The bar for the age group 35 to 44 represents 13 million volunteers.
The bar for the age group 25 to 34 represents 9 million volunteers.
The difference between the number of volunteers is:
$=13-9=4$ million
Thus, there are around $4$ million more volunteers aged 35 to 44 than those who are 25 to 34 years old.